Induced Air Flow Rate Research Articles

Enhancement of natural ventilation rate and attic heat gain reduction of roof solar collector using radiant barrier

Presented in this paper are the experimental results on natural ventilation flow rate enhancement and attic heat gain reduction of a roof solar collector equipped with a radiant barrier (RB). Investigation was conducted using an open ended inclined rectangular channel with an RB. The RB was used on the lower plate while the upper plate was heated with constant heat flux intensity. The channel dimensions are 1.5×0.70×0.19 m. The slope of the channel was fixed at 30° from horizontal plane. Four heat flux (190.5, 285.7, 380.9 and 476.2 W m −2) and five air gap space (3, 5, 7, 9 and 11 cm) were considered. Data analysis was made to determine the free convection heat transfer coefficient and induced airflow rate using two dimensionless parameters, viz., Nusselt number ( Nu) and Reynolds number ( Re). The Nu and Re were correlated as a function of Ra sin 30 and channel aspect ratio defined as the ratio of air gap space to the channel length. The relations obtained were as follows: Nu = 0.371 ( Ra sin 30) 0.2223( S/ L) −0.0469 and Re = 191.68 ( Ra sin 30 ) 0.1213 ( S / L ) - 0.085 . When compared to a conventional roof solar configuration with gypsum board on the lower part, it was observed that the use of RB increased convective heat transfer and airflow rate by about 40–50%, thereby increasing heat transfer reduction through the lower plate by about 50%. The developed correlations are useful for the design of such open-ended channels like the roof solar collector for passive ventilation of houses.

PERFORMANCE ANALYSIS OF CYLINDRICAL TYPE AIR EJECTOR

Performance tests were conducted for an air ejector entraining two secondary jets whose axes are coaxia l of that of primary stream. A unique ejector housing was constructed to receive both the convergent or convergent-divergent primary nozzle a nd the cylindrical mixing chamber. The air ejector under investigation has nozzle throat -to- cylindrical section area ratio of 0.12. The investi gation is conducted for ejector having cylindrical length-to-diameter ratio , which varies from 2.31 to 23.08. It covers a range of spacing from -1 to 0.5 of cylindrical lengths and of secondary to ambient pressure ratio of 1.0. The recorded axial distributions of the static wall pressure are plotted in non- dimensional forms and the extent of consistency of the profiles is shown. The induced air flow rate ratio is plotted for the measured primary to secondary stagnation pressure ratio range. Results offered a valuable appreciation of the effect of the cylindrical length and nozzle spacing-to- diameter ratios on both wall pressure distribution and jet entrainment capacity. The occurrence of flow choking in both th e convergent or convergent-divergent primary nozzle was ensured.

Investigation on thermal performance of glazed solar chimney walls

This paper reports investigation on the thermal performance of glazed solar chimney walls (GSCW) under the tropical climatic conditions of Thailand. The GSCW consisted of double glass panes with an air layer and openings located at the bottom (room side glass pane) and at the top (ambient side glass pane). A prototype of GSCW was integrated into the southern wall of a small room of 2.8 m 3 volume. Its dimensions were as follows: 0.74 m height, 0.50 m width and 0.10 m air gap. The size of openings was 0.05 × 0.5 m 2. With a clear glass of 6 mm thickness, velocity field measurement indicated that the induced airflow rate was about 0.13–0.28 m 3/s. The temperature difference between room and ambient was less than that with a single layer clear glass window. The reduction of daylight due to the double glass layer is negligible. Comparison between simulated and experimental results showed a reasonable agreement, therefore, the developed numerical model is valid and could be used as a tool for the design of GSCW.

Effect of Screening Solar Chimneys on Induced Air Flow Rate

This paper reports on a field investigation of the effect of screening on the induced flow rate in solar chimneys. The solar chimney considered here was a roof solar collector (RSC). It is composed of CPAC monier concrete tiles, an air gap and gypsum board. Two RSC units were integrated into the south-facing roof of a centre single room house of 25 m3 volume. Different types of screening were considered, namely: mosquito net, square net, metal grille and nylon filter. Different arrangements of one or more screens were installed at the top of one RSC unit while the other unit was not screened and served as a reference.It can be concluded that the addition of screens decreased the induced air ventilation depending on the remaining free surface area of the opening. The smaller the remaining free surface area of the opening, the higher the percentage decrease of air flow rate. However, the induced air flow rate is still relatively important to ensure sufficient space ventilation and indoor air motion. Based on experimental data, a simple linear relationship between the percentage of remaining opening area and the percentage decrease or drop of air flow rate was derived. The use of a nylon filter yields the highest rate of decrease (about 8%). When mosquito net, grille and nylon filter were used together, the decrease of air flow rate was about 18 %. However, when the wind speed is important, the drop of air flow rate is less significant.Finally, since screening (particularly the nylon filter) decreases the rate of dust admitted into the house with the incoming ambient air, it will play an important role in promoting the concept of the solar chimney ventilated house (SCVH)

Field study of performance of solar chimney with air-conditioned building

The study examines the performance of a solar chimney (SC) within an air-conditioned building. To this end, a single-room house of 25 m 3 volume was used. Two configurations of SC were used: the roof solar collector (RSC) composed of CPAC monier concrete tile, 14 cm air gap and gypsum board, and the modified trombe wall (MTW) composed of a masonry wall, 14 cm air gap and gypsum board. To control the induced air flow rate, as excess incoming hot ambient air will increase air-conditioner (AC) load, the size of the SC inlet opening was used as a means of ventilation control and three opening sizes were considered per each SC configuration. Experiments were performed throughout a period of six months (March–September). A split type AC of 1 t nominal capacity was installed. Comparisons between a common house and a solar chimney house (SCH) conducted using days with relatively similar ambient conditions demonstrated that a SCH could reduce the average daily electrical consumption of an AC by 10–20%, the ventilation fan saving must also be added. The appropriate size of inlet openings of a SC is 5×5 cm 2, which induces about 3–8 m 3/h/SC unit. The SC is very efficient for decreasing the AC load, namely when the AC is turned on at the beginning of afternoon (1 p.m.) as no heat is stored inside the room. In this operating mode the saving is much higher, about 30%. Consequently, the SCH is highly suitable for a hot climate as it could be used for both an AC and a non-AC building. The design recommendations formulated here could be used to conduct a realistic design depending on the requirements of the house’s owner.

Field measurements of performance of roof solar collector

To reduce the mechanical cooling energy cost of new housing built in a hot and humid region, the design should maximize the natural ventilation and minimize the fraction of sun energy absorbed by a dwelling. This objective is accomplished by using the roof structure to act as a solar collector. The roof solar collector design (RSC) used CPAC Monier concrete tiles and gypsum board. Two units of RSC were integrated in the roof structure of the school solar house. The effects of air gap and openings of RSC on the induced air flow rate and thermal comfort were studied experimentally.

Improving night ventilation into low-rise buildings in hot-arid climates exploring a combined wall–roof solar chimney

A theoretical investigation of a combined wall–roof solar chimney to improve night time ventilation in buildings is presented. A spreadsheet computer program is used for the parametric study to find out the optimum configuration of the wall–roof chimney. It has been reported that a roof solar chimney alone can induce an air flow rate of 0.81 m 3/s when the average incident solar radiation is 850 W/m 2. The maximum air velocity induced is 1.1 m/s when the 25° inclined chimney plates are 0.25 m apart. The aim of the paper is to predict the induced air flow rate as a result of the combined effect and to find the best height. The wall chimney height is varied from 1.95 to 3.45 m to determine the optimum length in relation to the chimney inlet. The results show that the air flow rate is three times more than that of the roof solar chimney alone (0.81 to 2.3 m 3/s). The maximum air flow rate of 2.3 m 3/s occurs at 3.45 m wall height. ACTION Psychrometrics Software (Sunshine Technology, USA, 1995) is used to predict the mean cooling load corresponding to the induced ACH. An air change per hour up to 26 could be achieved for a flat volume (321 m 3). Such ACH could be utilized to improve night ventilation to reduce indoor air temperature and cool low-rise heavy buildings with large diurnal outdoor temperature variations.

98/01401 Dynamic thermal model for predicting solar plant adequate energy management: Chaabene, M. and Annabi, M. Energy Convers. Mgmt, 1998, 39, (3/4), 349–355

To reduce the mechanical cooling energy cost of new housing built in a hot and humid region, the design should maximize the natural ventilation and minimize the fraction of sun energy absorbed by a dwelling. This objective is accomplished by using the roof structure to act as a solar collector. The roof solar collector design (RSC) used CPAC Monier concrete tiles and gypsum board. Two units of RSC were integrated in the roof structure of the school solar house. The effects of air gap and openings of RSC on the induced air flow rate and thermal comfort were studied experimentally.